It is well known by those versed in the art of radar pulse waveform design that radar target resolution is inversely related to radar waveform bandwidth. It is also well known that the optimum theoretical detection performance of any radar system is dependent only upon the pulse waveform energy and the receiver noise configuration. In an effort to increase waveform energy using peak power limited radar transmitters, long pulse duration, constant envelope and wide bandwidth signals are used. These efforts are covered by the general category of large time-bandwidth product or pulse compression waveforms and are disclosed by the following references: Huttman German Patent Ser. No. 768,068; Cauer German Pat. No. 892,772; Sproule, et al. British Patent Ser. No. 604,429; Dicke U.S. Pat. No. 2,624,876; and Darlington U.S. Pat. No. 2,678,997.
In general, the long duration, high energy radar pulse in the above-mentioned systems is phase (or frequency) modulated (or coded) to realize a bandwidth that is orders of magnitude greater than that predicted by its pulse width alone. And it is this phase (or frequency) modulation (or code) that must be removed by a receiver of the system when its received echoes are processed. To do so, the received long duration pulse is compressed by the receiver into a narrow, high amplitude pulse. In most existing systems, this compression is performed in a fixed analog dispersive delay line. However, in accordance with copending application Ser. No. 196,579 entitled "FM Modulation Technique for Producing Frequency Rejection Bands" by Cermignani, et al., and also copending application Ser. No. 196,578 entitled "Narrow Band Interference Suppressor for Pulse Compression Radar," by Schreiber, et al., both applications having been assigned to the same assignee as the instant invention, it may presently be performed digitally, using a real time programmable discrete Fourier transform/inverse Fourier transform device. The discrete Fourier transform of the received time waveform is taken in real time, conjugate phase weighted to cancel the phase modulation (or code), amplitude weighted to control temporal sidelobes or ambiguity, and then transformed back into the time domain.
There exists, however, electromagnetic environments under which such radar systems must operate where narrow bandwidth, high-power interference sources are active at frequencies within the same bandwidth as that of the pulse compression radar. If, as presently done, the combination of the received interference and the desired, small echo signal is processed by an analog dispersive delay line, the resulting compressed time pulse may become distorted and undetectable, due to the presence of the much larger interference.
One approach to correct this problem is to design and implement narrow, fixed bandwidth, band eliminate filters in the radar receiver, prior to pulse recompression, so that the unwanted interference frequencies are attenuated prior to passing the received echo signal through the dispersive delay line. Yet because the interference changes its center frequency and bandwidth as a function of time and radar antenna azimuth angle, the narrow band eliminate filters must track the interference. Consequently, the radar must perform a spectral analysis of the environment; that is, precisely locating the interference emitters in the radar band of operation and tuning the band eliminate filters to the undesired emitter center frequency.
In practice, however, since the narrow, fixed bandwidth, band eliminate filters would attenuate, besides the interference and noise, the signal itself, there is a significant net loss in the signal-to-noise ratio, especially if multiple narrow band cancellers are needed to remove multiple in-band interferences. As is well known in the signal processing art, this result follows directly from the fact that the resulting receiver transfer function is not the "matched filter" for the transmitted signal; hence, there is the degradation in post-detection signal-to-noise ratio caused by the mismatch filter loss.
Degradation in post-detection signal-to-noise ratio notwithstanding, the narrowed, fixed bandwidth, band eliminate filters also introduced intolerable increases in the compressed pulse temporal sidelobes. As is well known in the signal analysis art, this increase in the compressed pulse temporal sidelobes follows directly from the fact that receiver transfer function causes "paired echo" distortion of the recompressed pulse, i.e., the amplitudes of the paired echoes are proportional to the relative bandwidth of the band eliminate filters, and their locations relative to the main pulse of the signal are determined by their displacement from the center frequency of the original signal. Oftentimes this degradation of main lobe to sidelobe ratio is referred to as ambiguity, since there would appear to be many targets when in fact there is only one. Thus, present systems do not adapt the transmitted signals to avoid the interference bands but only filter the unadapted receiver signal to eliminate the interference, with resulting loss in detectability and distortion that causes loss of resolution and increases ambiguity.